Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising
  • C23C8/16—Oxidising using oxygen-containing compounds, e.g. water, carbon dioxide
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated

Definitions

• the present invention relates to a method for increasing the resistance to thermal shocks in the oxide layer and therewith for improving the oxidation behavior of heat conductive materials.
• Metallic materials which contain 3 to 10% aluminum and 10 to 25% chromium, as well as one or more reactive elements of the row of silicon and/or zirconium and/or hafnium and/or titanium, in an amount less than 5%, and/or one or more of the rare earth elements in an amount less than 0.3%, and/or alkaline earth metals of the group Mg, Ba, Ca, Sr and Be in an amount between 0.001 and 1% as well as the trace elements normally present in steels, the remainder of the alloy being iron and/or nickel and/or cobalt.
• the oxide layer produced on such alloys is designed to be rough so that it can also function in an advantageous manner as an adhesive base for further coatings, e.g. also for usage as catalytic carrier.
• These alloys have improved oxide layer properties (see Straford, K. N., "High Temperature Corrosion of Alloys Containing Rare Earth of Refractory Elements: A Review . . . ", High Temperature Technology, Vol. 1, No. 6, November 1983). In these alloys, the adhesion of the oxide layer is improved and thus the rate of oxidation is decreased.
• oxides of the rare earths such as Y 2 O 3 , which are especially finely dispersed in a base alloy, exert a similar, improving influence (See Ramanarayan, T. A., Raghavan, M. and Petkovic-Luton, R., "The Characteristics of Alumina Scales Formed on Fe-Based Yttria-Dispersed Alloys", J. Electrochem. Society, April 1984, Vol. 131, No. 4, pp. 923-931).
• Shell-shaped oxide can be produced in a known manner by special heat treatments on the surface of metallic materials from the latter.
• published European Patent Application EP-A No. 009156 describes how whisker-shaped oxides can be produced from ferritic steels containing more than 0.002% of rare earths if they are exposed to a long-lasting oxidation in preferably dry air at approximately 900° to 930° C.
• a similar state of the art is also described in British Patent No. 2,063,723.
• the disadvantage of this technique resides in the necessity of having to add rare earths in order to increase the adhesive strength of the different types of oxide layers of the alloy of the metal.
• Rare earths are not only expensive but they also react in the course of the manufacturing process of the semi-finished product with oxygen, impurities and the crucible materials so that high losses arise.
• the object of the present invention is to provide a method with which the adhesive strength of the oxide layer of heat-resistant steels which contain chromium and aluminum can be improved in such a manner that the content of rare earth metals can be reduced or eliminated for many applications.
• an alloy which contains 3% to 10% aluminum, 10% to 26% chromium, up to 3% zirconium and/or titanium and/or hafnium and/or niobium and/or silicon, and/or 0.002% to 0.3% in sum of rare earths and/or yttrium in metallic form or as finely dispersed oxides, remainder iron and/or nickel and/or cobalt as well as the trace elements customary in steels.
• the process consists of first heating the materials or components consisting of the materials in an oxygen-free atmosphere under conditions which cause recrystallization in their surface zone (thermal etching). Then, the materials can be oxidized, preferably in an atmosphere containing oxygen in chemically bound form which atmosphere contains a maximum of 1% free molecular oxygen.
• the oxide layer should consist, e.g. in the case of material described on page 120 of the annex Stahl-Eisen-Liste (Steel-Iron-List), (1977) 1.4767 which contains approximately 5% aluminum, of more than 96% aluminum oxide.
• This is achieved in accordance with the invention in that the material is heated at 700° to 1350° C. in an atmosphere containing oxygen in a chemically bound form.
• the atmosphere can consist of hydrogen-water vapor mixtures or of a mixture of these gases together with carbon dioxide and carbon monoxide, e.g. flue gas having a reducing composition. It is preferable to use carbon dioxide with as little oxygen as possible from which a carbon dioxide-carbon monoxide mixture forms during the oxidation.
• step (2) A thin layer of almost pure aluminum oxide is formed in step (2). Further absorption of oxygen is considerably retarded in the case of semi-finished products and components whose use temperature is e.g. approximately 700° to 950° C.
• Steps (2) and (3) can also be carried out with the aid of flue gas which is weakly reducing, e.g. the gas from an acetylene burner.
• the heat conductive materials contain small amounts of zirconium, titanium or hafnium. It is preferable to use 0.1 to 0.2% zirconium and titanium (0.1 to 0.15%). A further improvement is achieved if rare earths or alkaline earth metals are present in the alloy used as a starting material. In this manner, the questionable use of rare earths can frequently be avoided.
• the method can be carried out in a suitable vacuum furnace in combination with the oxidizing treatment in flue gases set to be reducing.
• a suitable vacuum furnace in combination with the oxidizing treatment in flue gases set to be reducing.
• One chamber of the furnace is used for the high-temperature treatment under a vacuum and other furnace chamber is used for the oxidation with the aid of chemically bound oxygen. In this manner, the method can be carried out in a single cycle.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Catalysts (AREA)
• Conductive Materials (AREA)

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Cited By (6)

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Citations (9)

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• 1988
  • 1988-02-12 DE DE3804359A patent/DE3804359C1/de not_active Expired
• 1989
  • 1989-01-14 EP EP89100624A patent/EP0327831A3/de not_active Ceased
  • 1989-02-09 US US07/308,211 patent/US4969960A/en not_active Expired - Fee Related

Patent Citations (9)

Non-Patent Citations (2)

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